Liquid crystal device alignment

ABSTRACT

Liquid crystal devices are formed by a layer of a liquid crystal material enclosed between two cell walls, both carrying electrode structures, and one or both walls treated to align molecules of the liquid crystal material. Most alignment treatment give alignment and surface pretilt with a strong azimuthal and zenithal anchoring energy to contacting liquid crystal molecules. The invention reduces at least one of the azimuthal zenithal or translational anchoring energy to improve switching characteristics and optical performance by allowing movement of liquid crystal molecules at or close to the cell wall. The reduction of anchoring energy may be achieved by an oligomer or short chain polymer which is either spread on the surface or added to the liquid crystal material. The size of oligomer or short chain polymer is low enough that it does not appreciably phase separate from the liquid crystal material. The polymer layer has the characteristics of having imperfect solubility in the liquid crystal material used in the device, of having a physical affinity for the surface of the substrate, and of retaining a substantially liquid like surface at the polymer/liquid crystal interface. The polymer may be formed by polymerisation of reactive low molecular weight materials in solution in the liquid crystal fluid. The resulting solution or dispersion of polymer in liquid crystal is then filled into the cell, and the polymer is allowed to coat the substrate surfaces.

This invention relates to liquid crystal device alignment.

Liquid crystal devices typically comprise a thin layer of a liquidcrystal material contained between cell walls or substrates. Opticallytransparent electrode structures on the walls allow an electric field tobe applied across the layer, causing a re-ordering of the liquid crystalmolecules.

Many different modes of liquid crystal devices are known in the art, forexample the twisted nematic device the cholesteric phase change device,the dynamic scattering device, the supertwisted nematic device and thesurface stabilised ferroelectric device modes. It is well known in allof these device modes to provide a surface on the interior walls of thedevice which will control the alignment of the liquid crystal fluid inclose proximity to the surface. For many applications of liquid crystaldevices, such a treatment is considered necessary in order to impose aparticular configuration on the alignment of the liquid crystal fluidthroughout the device and/or to provide an optical appearance in thedevice which is free of apparent optical defect. The particularsignificance of this factor for different classes of liquid crystaldevice is described in greater detail below.

The terms azimuth or azimuthal is used hen in to define the molecular(or director n) alignment angle movement or energy in the plane of thesubstrate surface. The terms zenith or zenithal is used herein to definethe molecular alignment angle movement or energy in a plane normal tothe substrate surface.

In respect of use of nematic and long pitch cholesteric materials fordevices known as twisted nematic liquid crystal devices, the relevanceof alignment and the problems associated therewith are as follows.

In order to provide displays with a large number of addressable elementsit is common to make the electrodes as a series of row electrodes on onewall and a series of column electrodes on the other cell wall. Theseform typically an x,y matrix of addressable elements or pixels and fortwisted nematic types of device are commonly addressed using rmsaddressing methods.

Twisted nematic (TN) and phase change devices are switched to an ONstate by application of a suitable voltage and allowed to switch to anOFF state when the applied voltage falls below a lower voltage level,i.e. these devices are monostable. For a twisted nematic type of device(90° or 270° twist as in U.S. Pat. No. 4,596,446) the number of elementsthat can be rms addressed is limited by the steepness of a devicetransmission verses voltage curve (as described by Alt and Pleschko inIEEE Trans ED vol ED 21, (1974) P.146-155). One way of improving thenumber of pixels is to incorporate thin film transistors adjacent toeach pixel; such displays are termed active matrix displays.

An advantage of nematic types of devices is the relatively low voltagerequirements. They are also mechanically stable and have a widetemperature operating range. This allows for the construction of smalland portable battery powered displays. An alternative twisted nematicdevice is one which is switched from a non-twisted state at zero voltsto a twisted state at a higher voltage, as described in GB 9607854.8,which will be referred to in this patent as a VCT device.

One problem with the twisted nematic device is that the contrast ratioof a normally white display remains at a low value until the voltage isincreased to a value considerably higher than the threshold voltage.This is due to the nematic material close to the cell walls which doesnot fully reorient in the applied field due to the strong zenithalanchoring imposed by the surface alignment layer. This lack of surfacereorientation also leads to higher voltage operation in the VCT device.

In respect of use of nematic and long pitch cholesteric materials fordevices known as bistable nematic liquid crystal devices, the relevanceof alignment and the problems associated therewith are as follows.

As described above, twisted nematic and phase change type of liquidcrystal devices are switched to an ON state by application of a suitablevoltage, and allowed to switch to an OFF state when the applied voltagefalls below a lower voltage level, i.e. these devices are monostable. Anadvantage of nematic type of devices is that they have relatively lowvoltage requirements. They are also mechanically stable and have widetemperature operating ranges. This allows for the construction of smalland portable battery powered displays. A disadvantage of such devices isthat their monostable switching characteristic limits the number oflines that can be multiplex addressed.

Another way of addressing large displays is to use a bistable liquidcrystal device. Ferroelectric liquid crystal displays can be made intobistable devices with the use of smectic liquid crystal materials andsuitable cell wall surface alignment treatment. Such a device is asurface stabilised ferroelectric liquid crystal device (SSFELCDs) asdescribed by:- L J Yu, H Lee, C S Bak and M M Labes, Phys Rev Lett 36,7, 388 (1976); R B Meyer, Mol Cryst Liq Cryst. 40, 33 (1977); N A Clarkand S T Lagerwall, Appl Phys Lett, 36, 11, 899 (1980). One disadvantageof ferroelectric devices is the relatively large voltage needed toswitch the material. This high voltage makes small portable, batterypowered displays expensive. Also these displays suffer from otherproblems such as lack of shock resistance, limited temperature range andalso electrically induced defects such as needles.

If a bistable switching characteristic can be achieved using nematicsthen a display can be made which has the merits of both the abovementioned technologies but without their problems.

It has already been shown by Durand et al that a nematic can be switchedbetween two alignment states via the use of chiral ions or flexoelectriccoupling: A Charbi, R Barberi, G Durand and P Martinot-Largarde, PatentApplication No WO 91/11747, (1991) “Bistable electrochirally controlledliquid crystal optical device”, G Durand, R Barberi, M Giocondo, PMartinot-Largarde, Patent Application No WO 92/00546 (1991) “Nematicliquid crystal display with surface bistability controlled by aflexoelectric effect”.

U.S. Pat. No. 4,333,708 describes a multistable liquid crystal device inwhich cell walls are profiled to provide an array of singular points.Such substrate configurations provide multistable configurations of thedirector alignments because disclination must be moved to switch betweenstable configurations. Switching is achieved by application of electricfields

Patent Application No:. WO97/14990, (PCT-96/02463, GB95 21106.6)describes a bistable nematic device having a grating surface treatmentto at least one cell wall that permits nematic liquid crystal moleculesto adopt either of two pretilt angles in the same azimuthal plane. Thecell can be electrically switched between these two states to allowinformation display which can persist after the removal of power.

Another bistable nematic device is described in GB.2,286,467-A. Thisuses accurately formed bigratings on at least one cell wall. Thebigrating permits liquid crystal molecules to adopt two differentangular aligned directions when suitable electrical signals are appliedto cell electrodes e.g. dc coupling to flexoelectric polarisation asdescribed in Patent Application No. WO.92/00546. Since in the twosplayed states the director is quite close to being in the plane of thelayer, the coupling between director and flexoelectric component can besmall, which may hinder switching in some circumstances.

The bistable nematic device of GB2286467-A also has a further problemwhich is not present in ferroelectric devices, that is, the need toswitch the surface layer of molecules in order to eliminate imagesticking effects. Surface layer switching usually requires high voltageswhich leads to both high power consumption and the need for customiseddriver circuitry.

In respect of devices using smectic materials, the relevance ofalignment and the problems associated therewith are as follows.

There are a number of devices based on smectic liquid crystal materialsincluding:

A: Ferroelectric liquid crystals (usually SMC*).

One example of this is bistable, and is often termed a surfacestabilised FLC device (SSFLC ref. N. A. Clark and S. T. Lagerwall, Appl.Phys. Lett., 36, 899 (1980). In this device planar aligned surfaces arearranged with parallel or anti parallel preferred alignment directions.The device is cooled from an overlying Smectic A phase into a bookshelfarrangement of the smectic layers, that is the material forms into microlayers arranged normal to the cell walls as in books on a shelf.

In the original teaching, the device used an unrubbed polymer surfacealignment treatment to ensure the liquid crystal director n liespreferentially and substantially parallel to the surface plane (i.e. ⊥to s, the surface normal). A preferred direction was then imparted byheating to the smectic A phase and shearing the layers in the requireddirection. The layers remained fixed on cooling into the SmC* phase. Thesurface energy is minimum for n ⊥ s so that two minimum energy statesoccur which can be selected by a suitable DC electric field.

Bradshaw and Raynes realised that improved SmA alignment for such adevice resulted by having a chiral nematic N* phase above the SmA inwhich the pitch was sufficiently long for the surface forces to causeunwinding of the spontaneous helicity for a significant temperaturerange above the transition. They also required that the surface shouldbe pre-treated to impart the preferred directions, often by use ofparallel or antiparallel rubbing of a polyimide or polyamide layer;GB-2,210,469 U.S. Pat. No. 4,997,264, GB-2,209,610, U.S. Pat. No.5,061,047, GB-2,210,468.

Later it was found that when a bookshelf aligned (where the layer normalis parallel to the plane of the dance i.e. δ=0) SmA sample is cooledinto the SmC* phase, the layers become tilted in a chevron type ofconfiguration; two type of chevron can exist and are defined as C₁ andC₂ type (ref J. Kanbe et al Ferroelectrics (1991) vol 114, pp3). Theseare shown in FIG. 18. This has been ascribed to the combined effect ofshrinkage of the smectic layer spacing and pinning of the layers at thesurface. The resulting chevron structure means that the director in themiddle of the cell is (roughly) fixed in one of two orientationssignificantly less than the full cone angle. This means that with noapplied field there is a substantial drop in the angle between the opticaxis of the two “surface stabilised” states which leads to acorresponding drop of the display brightness. A number of methods ofimproving the optical brightness have been proposed for practicaldevices:

1. AC field stabilisation:

An applied AC field pulse of insufficient time and voltage (τ V) tolatch into the two states couples to the dielectric tensor (primarilythe dielectric biaxiality) to increase the angle between these statesand enhance the brightness. The main problem with this type of approachis that a high frequency voltage is constantly required to maintain therequired brightness. This causes a high power dissipation, particularlyfor complex displays where the applied frequency is high. Usually thebrightness is compromised by using a suitably low AC voltage. It has theadvantage that, if C2U type alignment is used, there is no need forsurface switching, and hence surface memory effects are minimal, and theslower switching at the surface does not affect the device.

2 High pre-tilt Parallel:

This geometry has (approximately) the same chevron structure with thedirector at the chevron interface also at a low angle to the rubbingdirection. However, the director at the surface is at a much higherin-plane twist angle due to the competing effects of lying on the SmC*cone and with the preferred alignment pre-tilt. This type of devicegives good brightness but suffers from a slower response since itinvolves surface switching, and from strong surface memory problemswhich may lead to image sticking.

3 Quasi-bookshelf:

Two methods may be used to reduce the layer tilt angle and therebyincrease the device brightness. Pre-treating the device with a lowfrequency field of sufficient magnitude or choosing certain materials inwhich the layer shrinkage on cooling through the smectic phases isreduced (some materials may actually increase layer spacing on cooling).Such a device has similar advantages and disadvantages to the highpre-tilt configurations.

4 Uniform Tilted layer (High pre-tilt anti-parallel) geometry:

Similar to the previous two geometries, but there is no chevron (andtherefore no constraint on the director at the cell centre) and the highangle between the bistable states is stabilised solely by the surfaces.

B: Electro-clinic optical shutters:

Application of a DC field to the smectic A (or other orthogonal smectic)phase of a chiral material leads to an induced tilt of the director andhence optical axis normal to the applied field. In a (approximately)planar aligned liquid crystal cell with electrodes on the substratesurfaces the electroclinic effect induces a rotation of the optic axisby an angle proportional to the applied field E. Thus, an opticalshutter with full analogue amplitude or phase modulation may beobtained.

A common problem with such a device is obtaining suitably uniform andplanar alignment of the smectic layers. A lesser problem is that theinduced switching may involve some rotation of the director away fromthe preferred alignment direction at the surface. This movement issubject to a surface viscosity which may impede the switching time ofthe device and also to certain surface memory effects.

C: Anti-ferroelectric smectic liquid crystals (AFLC):

Certain materials form an anti-ferroelectric phase which may be used inactive matrix or direct drive devices. Effectively these devices have asimilar appearance to the smectic A phase until sufficient DC voltage isapplied, above which the sample is in either of two states (depending onthe polarity of the applied signal) similar to the normal ferroelectricphase.

There is a limited number of materials which form this phase(particularly over a wide temperature range) and all those found so farhave direct isotropic to smectic phase (i.e. no overlying chiral nematicphase). This means that the materials are more difficult to align,forming batonnets (see Gray and Goodby book) of the smectic at thistransition.

The mechanism for this is that the smectic layer structure nucleates ina limited number of “cold spots” in the isotropic liquid. The layersthen curve around this point to minimise the bend and splay of the layernormal. Where the layers meet the surface they become pinned anddifficult to move. Hence, it is difficult to obtain the desired layerarrangement (e.g. planar or bookshelf) once the batonnet structure haspre-formed. On cooling into the AFLC phase, the applied field tends toinduce twist of the director at the surfaces which also leads toproblems associated with surface switching such as slower speed, surfacememory effects, etc.

D: SmC* Optical Shutters:

Bradshaw and Raynes also described a type of device in which the FLC isobtained from cooling directly from the unwound No phase in a parallelrubbed device, preferably within applied DC field applied during thephase transition. The unwound N* phase has the director in the rubbingdirections and on cooling into the SmC* this orientation is maintainedand the layer normal twists through the angle θ. Degeneracy of thedirection in which the layer normal is oriented is removed by theapplication of the DC field.

This is a monostable device, since it always relaxes back to the surfacestabilised state (with n ∥ s) once the field is removed (it may be usedin devices when the field is retained, either through AC stabilisationor through inclusion of TFTs or similar non-linear electrical elementsat each pixel. However, it is fist (due to Ps). Primarily, switchingoccurs in the bulk of the cell and little or no switching occurs at thesurface. However, this means that the director is highly twisted and nonuniform in structure. This means that the optical appearance is poor(particularly if used in conjunction with a dye as done in early Hitachiwork) and so this is a case where surface switching is required toimprove performance. Also, alignment is difficult over a widetemperature range because layer shrinkage still occurs in many N-SMC*materials, leading to a chevron structure and associated defects.

Alignment of liquid crystals on a surface is therefore a significantproblem for all these device types. Several different means are known bywhich liquid crystal fluids may be aligned on a surface. Evaporation ofsilicon monoxide from a direction at least 30° from the plane of thesubstrate provides a surface which aligns a nematic liquid crystal inthe plane of the substrate, along an axis orthogonal to the evaporationdirection. In contrast, if the evaporation is conducted from a directionmaking an angle of about 5° or less from the substrate, the resultingsurface aligns a nematic liquid crystal along a direction tilted fromthe plane of the substrate by about 20° in the direction of theevaporation source.

Many commercial liquid crystal devices are fabricated using rubbedpolymer alignment layers, especially rubbed polyimide alignment layers.Typically such layers are deposited as an amide precursor polymer byspin deposition of a solution. After removal of the solvent, the polymercoating is imidised by baking at high temperature, then unidirectionallynibbed with a cloth. The resulting surface aligns liquid crystalmaterials along the direction of rubbing with a tilt out of the plane ofthe surface in the direction of rubbing. The magnitude of the tilt angleis typically 1° to 2°, but special polyimide formulations and treatmentsare available which can provide higher magnitudes of pretilt. Somepolymer layers are capable of aligning liquid crystal material whencross linked by exposure to linear polarised light (WO95/22075,GB-9444402516). This avoids the need for rubbing which is useful whensubstrates carry thin film transistors for a part of active matrixdisplays. The aligned polymer may also be used in conjunction withgratings as noted below.

A further means to provide a surface alignment for liquid crystalmaterials is available from the deposition of different surfactantmaterials onto the substrate from solution. A range of differentsurfactants may be used, including quaternary ammonium salts, alkylatedsilazenes and basic chromium alkanoates. Treatment of the surfaceusually entails dipping or spin coating with a dilute solution of thesurfactant, and usually results in an alignment of the liquid crystalorthogonal to the plane of the substrate, termed homeotropic alignment.Binuclear chromium alkanoates and other binuclear surfactants mayprovide alignment in the plane of the substrate without any preferreddirection in this plane.

Yet, a further method to achieve liquid crystal alignment at a surfaceinvolves fabrication of a relief structure such as a relief grating onthe surface. Such a structure may be obtained by photolithographicmeans, by embossing a compliant surface layer such as a polymer againsta master structure fabricated on, for example, a metal sheet, bymechanically scribing the surface or by other means. A grating structurealigns a nematic liquid crystal along the direction of the troughs andcrests of the grating. More complex relief structures can provide tiltedor bistable alignment.

The alignment methods of the known art suffer a number of shortcomingswhich prevent liquid crystal devices manufactured according to thesemethods from achieving their full potential utility.

One such shortcoming is that it is hardly possible according to knownmethods, to provide a surface alignment treatment on which the liquidcrystal alignment is free to adopt any alignment direction in the planeof the surface. A planar alignment may be obtained by various methodsincluding evaporation of an inorganic material from substantially normalincidence to the substrate, or by coating the substrate with a knownpolymer material such as a polyimide material without mechanicalrubbing. In these cases, the alignment of the liquid crystal on thesurface is not fixed during the surface preparation, but is fixed by thealignment of the liquid crystalline phase which first contacts it, andthen becomes immovable.

On such a surface the alignment direction is determined by such factorsas the flow direction or the direction of a temperature gradient orelectric fields at the time the liquid crystal phase first contacts thesurface. It is desirable to provide a surface treatment which can allowthe liquid crystal alignment direction to rotate freely and repeatedlyin the surface plane, but this is not available from known surfacetreatments.

A second shortcoming of known liquid crystal alignment techniques isthat the energy required to change the zenithal angle between thesubstrate and the liquid crystal director is much greater than theelastic distortion energy of the liquid crystal itself which isgenerated by commonly applied voltages. This means that in liquidcrystal devices using known alignment techniques, the liquid crystaldirector remains substantially fixed in tilt angle at the cell walls andthe switching of the device which provides an optical effect occurringonly in the parts of the device which are separated from the cell wallsby some distance which depends on the magnitude of the applied field.

The present inventors have found that the above problems are reduced bya surface alignmlent treatment which allows movement of liquid crystalmolecules at or close to the cell walls, hence the liquid crystaldirector is in contact with the wall to reversibly change itsorientation at low values of applied field, for example at applied fieldstrengths of the order of less than 1 volt per micron for an appliedelectric field. The benefits of such a surface treatment may includereduction in the operating voltage of the device and/or an improvementin the switching behaviour of the device such as the electro-opticthreshold steepness of the device which determines the amount ofinformation which may be written on an electro-optic display by means ofthe known methods of RMS multiplex driving.

Accordingly, in a first aspect the invention provides a liquid crystaldevice comprising a layer of a liquid crystal material contained betweentwo spaced cell wall carrying electrodes structures and an alignmenttreatment on at least one wall, characterised by means for reducinganchoring energy at the surface alignment on one or both cell walls.

The anchoring energy reduced is one or more of azimuthal anchoringenergy, zenithal anchoring energy, and translational anchoring energy(movement along the alignment treated surface). The significance ofanchoring energies in the context of different device types arediscussed further below. Further aspects of the invention relevant tospecific device types are also discussed further below.

Anchoring energy arises from surface topography features such as groovesor gratings, and from chemical bonding interactions. The presentinvention reduces anchoring energy by changing the chemical bonding.Additionally the surface topography may also be changed, for example toreduce the dimensions of grooves or gratings. The means for reducingenergy may be an oligomer or short chain polymer which is either spreadon the surface or added to the liquid crystal material. The size ofoligomer or short chain polymer may be selected to give a desired amountof preferential deposition at cell walls and slight separation from theliquid crystal material host.

The means for reducing anchoring energy may be an oligomer containingesters, thiols, and/or acrylate monomers and or which is either spreadon the surface or added to the liquid crystal material.

The alignment treatment and means for reducing anchoring energy may beprovided by a double layer treatment, now referred to as a substratelayer and a polymer layer. The substrate layer may either be formed inthe surface of the cell wall, e.g. by mechanical rubbing of the surface,or (and preferably) be a coating on the cell wall. This coating mayinclude anisotropic features which act to align liquid crystal phasesplaced in contact with it or in close proximity to it. Such features mayinclude surface relief features including a plain or blazed grating orbigrating structure, or a regular or irregular array of surface featuresincluding but not limited to columns, tilted columns, platelets andcrystallites e.g. formed by normal or oblique evaporation of inorganicmaterials onto the surface or by mechanical abrasion or working of thesurface. Such features may also include a substantial anisotropy in thesubstrate formed, for example, by mechanical stretching or rubbing ofthe substrate layer or by exposure of the substrate layer to polarisedactinic radiation.

The polymer layer (formed on the substrate layer) has thecharacteristics of having imperfect solubility in the liquid crystalmaterial used in the device, of having a physical affinity for thesurface of the substrate, and of retaining a substantially liquid likesurface at the polymer/liquid crystal interface.

The polymer may be applied to the device in various ways. In oneapproach, the polymer is formed by polymerisation of reactive lowmolecular weight materials in solution in the liquid crystal fluid. Theresulting solution or dispersion of polymer in liquid crystal is thenfilled into the cell, and the polymer is allowed to coat the substratesurfaces. Optionally, the dispersion of polymer in liquid crystal mayundergo intermediate processes such as filtration or centrifuging priorto being filled into the display cell.

In a further approach to applying the polymer to the device the reactivelow molecular mass materials may be dissolved into the liquid crystalwhich is then filled into the display cell. Polymerisation is theninitiated by known means, such as by heating or exposure to shortwavelength optical radiation in the presence of an initiator. Afterpolymerisation the polymer is allowed to diffuse to and coat thesubstrate layers.

A still further approach to applying the polymer to the device isprovided by polymerisation of the reactive materials in the presence orabsence of an inert solvent. The solvent, if present, is removed and theresulting polymer is dissolved in the liquid crystal and filled in tothe display cell.

A further approach to applying the polymer to the device is to form thepolymer on the substrate by applying a thin layer of reactive lowmolecular weight materials to the substrate by known means such as byspinning a stoichiometric amount of each onto the substrate in solutionin a solvent. After removal of the solvent, polymerisation is initiatedby heating or by exposure to light in the presence of a polymerisationinitiator. The treated substrates are then assembled into a cell and theliquid crystal added in.

The polymer is characterised in that it is substantially non-crystallinein the presence of the liquid crystal, and that it possesses a glasstransition temperature below the operating temperature range of thedevice. The polymer may be substantially linear in its molecularstructure or it may include branch points. The polymer may also becrosslinked to a low degree in order to promote phase separation fromthe liquid crystal and deposition onto the substrate, but suchcrosslinking is at such a level that a fluid, gum-like, gel-like orelastic character is retained, and the polymer does not present a hardglassy or solid like character which is retained on heating.

Preferred polymeric materials include thiol/ene polymers prepared byfree radical polymerisation of known monomers in the presence of anadded thiol compound which serves to limit the molecular weight of theproduct through chain transfer reactions. Details of suitable materialare listed later.

In relation to twisted nematic devices, the present inventors have foundthat the contrast ratio of a twisted nematic device can be improved byusing an additional a surface treatment which reduces the zenithalanchoring energy of the surface and thereby allows field-inducedreorientation of the near-surface nematic layers. Such a treatment alsohas the added advantage of leading to a lowering of the thresholdvoltage. Lower voltage operation is preferable for both passive matrixand active matrix twisted nematic devices as it allows a display tooperate with a lower power consumption.

Accordingly, in a second aspect the invention provides a twisted nematicliquid crystal device capable of being switched from a twisted state toa non twisted state comprising; two cell walls enclosing a layer ofnematic liquid crystal material; electrode structures on both walls forapplying an electric field across the liquid crystal layer; a surfacealignment on both cell walls providing alignment direction to liquidcrystal molecules and arranged so that a twisted nematic structure isformed across the liquid crystal layer at either zero volts or at ahigher voltage; means for distinguishing between the two differentoptical states of the liquid crystal material; CHARACTERISED BY meansfor reducing zenithal anchoring energy in the surface alignment on oneor both cell walls.

Additionally the azimuthal anchoring energy may also be reduced.

The means for reducing azimuthal anchoring energy and zenithal anchoringenergy may be an oligomer containing esters, thiol, and/or acrylatemonomers either spread on the surface or added to the liquid crystalmaterial, e.g. the materials N65 and MXM035.

The oligomers may migrate preferentially to the surface in order tominimise the surface free energy. This may dilute the amount of liquidcrystal at the surface leading to an effective reduction in the orderparameter, S which is defined by (P. G. deGennes, The Physics of LiquidCrystals, Clarendon Press, Oxford 1974):$S = {\frac{1}{2}{\langle\left( {{3\cos^{2}\theta} - 1} \right)\rangle}}$

The order parameter is an indication of how well molecules align in acell. Additionally the phase of the liquid crystal material at thesurface may be changed by the oligomers, e.g., from nematic or longpitch cholesteric to isotropic.

The treatment may be used in conjunction with a surface which inducesmonostable pretilted nematic alignment.

The alignment layer may be a rubbed polymer surface as described in S.Ishihara et al., Liq. Cryst, vol.4, no. 6. p.669-675 (1989) or anobliquely evaporated inorganic material as described in W. Urbach, M.Boix, and E Guyon, Appl. Phys. Lett., vol. 25, no. 9, 479 (1974) or apolymer surface where in-plane anisotropy is achieved by illuminationwith polarised light such as M. Schadt et al., Jpn. J. Appl. Phys., v.31, no.7, p.2155 (1992).

Alternatively, the alignment layer may be a surface-monograting with anasymmetric groove profile as described in G. P. Bryan-Brown and M. J.Towler, “Liquid crystal device alignment”, GB 2,286,466A (GB9402492.4).

The alignment directions on the two surfaces may be substantiallyperpendicular.

The nematic liquid crystal may contain a small amount (<5%) of a chiraldopant material e.g., R1011, CB15 Merck.

The cell walls may be substantially rigid e.g., glass material, orflexible e.g., polyolefin.

The electrodes may be formed as a series of row and column electrodesarranged and an x,y matrix of addressable elements or display pixels.Typically, the electrodes are 200 μm wide spaced 20 μm apart.

Alternatively, the electrodes may be arranged in other display formatse.g., r-θ matrix or 7 or 8 bar displays.

In relation to bistable nematic devices, the present inventors havefound that the problem of surface layer switching is reduced by using asurface treatment which changes the liquid crystal properties in thevicinity of the surface and so leads to a lower anchoring energy betweenthe liquid crystal and the surface. This allows lower voltage operationwithout compromising other device parameters.

Accordingly, in a third aspect the invention provides a bistable nematicliquid crystal device which comprises; two cell walls enclosing a layerof nematic liquid crystal material; electrode structures on both walls;a surface alignment on both cell walls providing alignment direction toliquid crystal molecules; means for distinguishing between switchedstates of the liquid crystal material; CHARACTERISED BY means forreducing inelastic azimuthal memory anchoring energy in the surfacealignment on one or both cell walls.

Ideally, the inelastic azimuthal memory anchoring energy is reduced tozero. Preferably, the zenithal anchoring energy is also reduced.

The means for reducing energy may be an oligomer or short chain polymerwhich is either spread on the surface or added to the liquid crystalmaterial.

Preferably, the oligomer or short chain polymer does not change thepretilt by a substantial amount, e.g., change the pretilt by more than5°.

The treatment is used in conjunction with a surface which inducesbistable nematic alignment.

The bistable surface may be a surface alignment bigrating on at leastone of the cell walls that permits the liquid crystal molecules to adopttwo different azimuthal alignment directions, as in patent applicationWO097/14990, (PCT-96/02463, GB95 21106.6).

The angle between the alignment directions may be 90° or less than 90°.

The grating may be a profiled layer of a photopolymer formed by aphotolithographic process, e.g., M C Hutley, Diffraction Gratings(Academic Press, London 1982) p 95-125; and F Horn, Physics World, 33.(Mach 1993). Alternatively, the bigrating may be formed by embossing; MT Gale, J Kane and K Knop, J App. Photo Eng, 4, 2, 41 (1978), or ruling;E G Loewen and R S Wiley, Proc SPIE, 88 (1987), or by transfer from acarrier layer.

The bigrating may have a symmetric or asymmetric groove profile. In thelatter case the surface induces both alignment and pretilt as describedin GB2286467-A.

The gratings may be applied to both cell walls and may be the same or adifferent shape on each wall.

The bistable surface could alternatively be formed by using an obliquelyevaporated material as described in patent Application WO 92/0054 (GDurand, R Barberi, M. Giocondo and P Martinot-Largarde, 1991).

The cell walls may be substantially rigid, e.g., glass material, orflexible e.g., polyolefin.

The electrodes may be formed as a series of row and column electrodesarranged and an x,y matrix of addressable elements or display pixels.Typically, the electrodes are 200 μm wide spaced 20 μm apart.

Alternatively, the electrodes may be arranged in other display formats,e.g., r-θ matrix or 7 or 8 bar displays.

In relation to smectic devices, the inventors have found that problemsin such devices may be reduced by use of a surfactant to lower theinteraction between the surface(s) of cell wall(s) and the liquidcrystal in the smectic phase, or in the overlying nematic phase fromwhich the cell is cooled into the smectic phase for all operatingtemperatures. This use of a surfactant may be termed a slippery surfacetreatment. Thus, improved alignment, optical properties, switchingspeed, and stability to shock of smectic devices are achieved throughslippery surface treatment.

Accordingly, in a fourth aspect, the invention provides a smectic liquidcrystal device which comprises: a liquid crystal cell including a layerof smectic liquid crystal material contained between two walls bearingelectrodes and surface treated to give both an alignment and a surfacetilt to liquid crystal molecules; CHARACTERISED BY means for reducinganchoring energy at the surface alignment on one or both cell walls.

The means for reducing anchoring energy may be an oligomer containingesters, thiols, and/or acrylate monomers and or which is either spreadon the surface or added to the liquid crystal material.

In its most elemental form the surfactant provides a slippery surfacewhich reduces the interaction between the liquid crystal molecules andthose of the surface of the cell wall (or alignment layer surface).Thus, the slippery surface may be thought of as having increased freedomfor translational and rotational movement of the liquid crystalmolecules closest to the surface. There are five surface terms (ref: IntFerroelectric Liquid Crystal Conf(FLC95), Cambridge, UK, Jul. 23-27,1995, vol.178 No.14 J. C. Jones, pp155-165) which are relevant and maybe controlled by the surfactant:

(1) α, zenithal anchoring energy. How easily the director surface tiltangle is changed (i.e., a rotational energy).

(2) β, azimuthal anchoring energy—case of changing surface twist angleof director (i.e., a rotational energy).

(3) γ, related to the pretilt angle of the director at the surface.

(4) Layer pinning term—How easily layers may be moved across the surface(i.e. a translational energy). This is the macroscopic effect of the(partial) adsorption of liquid crystal molecules onto the surface layerreducing translational movement of the molecules and hence of thesmectic layers.

(5) Polar surface energy—In ferroelectrics (or flexoelectrics) a termwhich is minimum for a particular orientation of the Ps at the surface.

In this aspect of the present invention each of these factors isinfluenced by the presence of a slippery surfactant which acts toseparate the solid and liquid crystal regions by the induced changes ofliquid crystal order close to the surface. For example, if nematic orderexists close to the surface layer of a smectic device, then layerpinning is greatly reduced. If the cone angle is lower, surfaceswitching is reduced, as well as the polar surface term.

Advantages provided by this aspect of the present invention are asfollows:

(1) Reduced layer pinning hence control of the smectic layers is easier;

(2) Reduced nematic-like surface energies, hence orientation changes ofthe director at the surface are enhanced.

(3) Reduced adsorption of liquid crystal molecules at the surface, hencereduced surface memory effects and reduced surface viscosity;

(4) Reduced polarity of the surface, hence less coupling to thespontaneous polarisation coefficient (Ps) in ferro electric liquidcrystal systems resulting in less T state formation.

Specific embodiments of the invention will be described below, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of a matrix multiplexed addressed liquid crystaldisplay;

FIG. 2 is the cross section of the display of FIG. 1;

FIG. 3 shows the configuration for photolithographic exposure leading tothe formation of an asymmetric monograting useful in twisted nematicdevices.

FIG. 4 shows the transmission versus voltage data for two twistednematic cells, one of which (dotted line) has been treated with anadditive (Norland 65) to give weak anchoring on asymmetric gratings.

FIG. 5 shows the optical contrast ratio versus voltage data for twotwisted cells, one of which (dotted line) has been treated with anadditive to give weak anchoring.

FIG. 6 shows transmission—voltage curves for two voltage controlledtwist type cells, one with a standard alignment, the other with a weakanchoring energy treatment.

FIG. 7 shows the configuration for photolithographic exposure leading tothe formation of a bigrating with orthogonal grating modulations.

FIG. 8 shows switching characteristics for two bistable cells, one withstandard alignment, the other with a weak surface anchoring energy.

FIG. 9 shows diagrammatically a smectic liquid crystal molecule, and howit moves within a layer when switched to its bistable states in aferroelectric liquid crystal cell.

FIG. 10 shows variation in memory angle against applied voltage for twobistable ferroelectric liquid crystal devices, one with standardalignment, the other with a weak surface anchoring energy.

FIG. 11 shows the theoretical surface director tilt versus voltage forvalues of surface extrapolation lengths L.

FIG. 12 shows the theoretical midlayer director tilt versus voltage forvalues of L.

FIG. 13 shows the transmission versus voltage data for two cells, one ofwhich (dotted line) has been treated with an additive (MXM035) to giveweak anchoring. Gratings have been used on both surfaces for alignment.Cell gaps are 2.05 μm.

FIG. 14 shows the transmission versus voltage data for two cells, one ofwhich (dotted line) has been treated with an additive (MXM035) to giveweak anchoring. Gratings have been used on both surfaces for alignment.Cell gaps are 4.6 μm.

FIG. 15 shows the transmission versus voltage data for two cells, one ofwhich (dotted line) has been treated with an additive MXM035) to giveweak anchoring. Rubbed polymer layers have been used on both surfacesfor alignment. Cell gaps are 4.6 μm.

FIG. 16 is a diagrammatic view of a bistable ferro electric display withrow and column drivers providing an x,y matrix display.

FIG. 17 is a cross section of the display cell of FIG. 16; and

FIG. 18 is a schematic view of a layer of ferro electric liquid crystalmaterial, showing two alignment configurations, the C₁ and the C₂states.

The application of aspects of the present invention to twisted nematic,bistable nematic, and smectic devices will now be described in separategroups of examples.

TWISTED NEMATIC DEVICES

The display in FIGS. 1, 2 comprises a liquid crystal cell 1 formed by alayer 2 of nematic or long pitch cholesteric liquid crystal materialcontained between glass walls 3, 4. A spacer ring 5 maintains the wallstypically 2-10 μm apart. Additionally, numerous beads of the samedimensions may be dispersed within the liquid crystal to maintain anaccurate wall spacing. Strip like row electrodes 6 e.g., of SnO₂ or ITOare formed on one wall 3 and similar column electrodes 7 are formed onthe other wall 4. With m-row and n-column electrodes this forms an m×nmatrix of addressable elements or pixels. Each pixel is formed by theintersection of a row and column electrode. A row driver 8 suppliesvoltage to each row electrode 6. Similarly, column driver 9 suppliesvoltages to each column electrode 7. Control of applied voltages is froma control logic 10 which receives power from a voltage source 11 andtiming from a clock 12.

On either side of the cell 1 are polarisers 13, 13′ arranged with theirpolarisation axes substantially crossed with respect to one another. Anadditional optical compensator such as a stretched plastic film may alsobe added between the liquid crystal cell and one of the polarisers. Apartly reflecting mirror 16 may be arranged behind the cell 1 togetherwith a light source 15. These allow the display to be seen in reflectionand lit from behind in dull ambient lighting. For a transmission device,the mirror may be omitted.

Prior to assembly, the cell walls 3, 4 are treated with alignmenttreatments to provide a monostable pretilted alignment. The alignmentdirections R1, R2 are shown as orthogonal to give a 90° or 270° twistedcell, but may be at other angles, e.g., at 45°. Finally, the cell isfilled with a nematic material which may be e.g., E7, ZLI2293 or MLC6608 (Merck), and may include a chiral additive such as CB15 or R1011(Merck).

In use, the display may be multiplex addressed in a conventional mannerby the application of a row waveform applied to each row in turn whilstapplying waveforms to all columns. Such addressing is capable ofapplying two different rms value waveforms at each x,y intersection. Onewaveform has an rms value above a switching threshold and will thereforeswitch the liquid crystal material to an ON state. The other resultantwaveform has an rms value below the switching threshold and thereforedoes not switch the liquid crystal material.

The number of x,y pixel elements that can be rms addressed is limited bythe steepness of a device transmission verses voltage curve (asdescribed by Alt and Pleshko in IEEE Trans ED vol ED 21, (1974)P.146-155). Therefore improvements to the steepness of thetransmission—voltage curve are highly desirable. Additionally, if moreof the material can be switched (switching the molecules adjacent a wallsurface, rather than switching only at the layer centre) then a highercontrast between ON and OFF states would be obtained.

These nematic materials in embodiments of the invention contain thetreatment or a precursor of the treatment which leads to a lowering ofthe anchoring energy.

In general, anchoring of a nematic liquid crystal on a surface can bedescribed by three macroscopic parameters, pretilt, zenithal anchoringenergy and azimuthal anchoring energy. Consider a surface in the x-yplane, parallel to the cell walls. The pretilt (θ_(p)) is defined as thepreferred angle of inclination of the nematic director with respect tothe x-y plane. To change the tilt of the surface director from θ_(p) toan arbitrary tilt θ; an energy per unit area of W must be supplied tothe system where [A. Rapini and M. Papoular. J. Phys. (Paris), 36, C-1,194 (1975)]:

W=W _(θ)sin²(θ−θ_(p))  (1)

W_(θ) is the zenithal anchoring energy and represents the energyrequired in order to change the tilt of the surface director by 90°. Ifthe director has a preferred in-plane orientation, say along the x axis,then an energy must be supplied to the system to change thisorientation. The energy is now given by

W=W _(φ)sin²φ  (2)

where φ is the change in the in-plane orientation and W_(φ) is theazimuthal anchoring energy.

Pretilt and zenithal anchoring can be achieved from most solid surfaceswhile azimuthal anchoring usually requires some extra treatment in orderto obtain a preferred in-plane direction such as an anisotropic polymer,obliquely evaporated film, or a surface grating. For most surfaces,W_(θ) end W_(φ) are large and so reorientation of the director at thesurface only occurs at high voltages.

The effect of the weak zenithal anchoring on a twisted nematic device isdiscussed below.

The improvement in operation of a 90° twisted nematic cell with weaksurface anchoring was first analysed theoretically. The staticconfiguration of the cell was calculated by minimising the total freeenergy which is dictated by the Euler-Lagrange equation in the bulk andby equations (1) and (2) at the surfaces. The zenithal anchoring energyW_(θ) was then relaxed from its usual large value and the effect on thestatic configuration was calculated. FIG. 3 shows several curvescalculated for different surface extrapolation lenghts, L whereL=k₁₁/W_(θ)·d. The parameters common to all the curves are;

k₂₂/k₁₁=0.6; k₃₃/k₁₁=1.5; ε_(para)=14.0; ε_(per)=4.0; Cell twist=90°;Surface pretilt=5°

The reduced voltage is defined as the voltage which has been normalisedby the Frederiskz threshold voltage, (={square root over(k₁₁/ε_(o)Δε)}). FIG. 11 shows the surface tilt angle as a function ofapplied voltage. For infinite zenithal anchoring energy (L=0.0) the tiltremains fixed at the zero volt pretilt value. However for finiteanchoring (L>0) all the curves show that the surface director isreoriented by the applied field and shows an increasing tilt angle withvoltage. It is expected that this voltage induced tilt will lead to abetter contrast ratio in a typical normally-white twisted nematicdevice.

FIG. 12 has been calculated with the same parameters as FIG. 11 but nowshows the voltage dependent tilt in the middle of the nematic layer.This tilt angle is the dominant parameter in dictating the opticaltransmission of the twisted nematic device. Therefore, it is clear tosee that a surface offering a finite L will lead to a lower voltage,steeper electrooptic response.

Therefore, the above modeling has shown that a surface treatment whichlowers W_(θ) will lead to a twisted nematic device which has a lowervoltage threshold, a steeper electrooptic response and a higher contrastat a given voltage.

EXAMPLE TN1

An example of a weak anchoring treatment applied to a twisted nematic isnow given. The pretilted alignment surface used in this example was anasymmetric monograting as described in GB 9402492.4; GB-A-2,296,466;WO-95/22078.

The treatment consists of adding a small (1-10%) amount of a UV curingadhesive material to the nematic prior to cell filling. Examples ofsuitable materials include N65, N63, N60 or N123 (All manufactured byNorland Products Incorporated, North Brunswick, N.J., USA). In thisparticular example, one of these materials (N65) is used as an additiveto the nematic E7 (Merck). This material contains a mixture of estersand acrylate monomers which polymerise under UV radiation.

Before using the N65 additive in a twisted nematic device a set ofexperiments were carried out in order to show the effect of the N65treatment on the zenithal anchoring energy, W_(θ). This quantity can becalculated by measuring the saturation voltage, V_(s). That is, thevoltage at which the director tilt in the cell is perpendicular to thesurface throughout the thickness of the cell. This can be measured incells where the surfaces have no preferred alignment direction. In thiscase flat surfaces of hardbaked photoresist were used (Shipley 1805).This material was spin coated on ITO coated glass to form a 0.55 μmthick layer. Baking at 160° C. for 45 minutes ensured full insolubilityin the liquid crystal. When filled with N65/E7 mixtures, these cellsshow a random Schlerien texture. The saturation voltage was measured byobserving when the transmitted intensity of the Schlerien texture fallsto zero when viewed between crossed polarisers. W₇₄ is then given by:$\begin{matrix}{W_{\theta} \approx \frac{3.85{\sqrt{ɛ_{o}\Delta \quad ɛ\quad k_{11}} \cdot V_{s}}}{d}} & (4)\end{matrix}$

where d is the liquid crystal thickness, k₁₁ is the liquid crystal splayelastic constant and Δε is the liquid crystal permittivity anisotropy.

Results are shown in table 1. The pure E7 cell failed to show a blackstate before cell breakdown and so only a lower limit on W_(θ) can begiven. In the cases of the E7 containing N65, the curing was performedin a fused silica cell for 10 minutes prior to transferring the mixtureto a separate measurement cell. The exposure was carried out using anunfiltered mercury lamp with an optical output of 2.0 mW/cm² at a raisedtemperature of 65° C.

TABLE 1 Cell mixture W_(θ)(N m⁻¹) Pure E7  >5 × 10⁻² 2% N65 in E7 6.3 ×10⁻³

Surface zenithal anchoring energies modified by the presence of N65.

The above results clearly show that the N65 has reduced the value ofW_(θ) in a cell with flat surfaces. The next step is to study the effectof this additive on the operation of a twisted nematic device.

Such a twisted nematic device may employ asymmetric monogratings toinduce pretilted alignment and be fabricated in the following way asshown in FIG. 3: Shipley 1805 photoresist 20 was spin coated at 3000 rpmonto ITO coated glass 21 for 30 seconds. Net the photoresist layer 20was baked at 90° C. for 30 minutes to remove the solvent. Exposure ofthe photoresist through a mask 22 was carried out using off axis hardcontact photolithography. The mask 22 consisted of a chrome on glasspattern with a pitch of 1 μm (0.5 μm gaps and 0.5 μm chrome strips). Theexposure time was set to 540 seconds with an incident power of 0.15mW/cm² from a mercury lamp. Development was then carried out in ShipleyMF319 for 10 s followed by a water rinse. Samples were finally baked at160° C. for 45 minutes after first receiving a deep UV exposure topreharden the photoresist (3.36 J/cm² at 254 nm).

The above process resulted in a surface monograting with a 1 μm pitchand a 0.5 μm peak to trough groove depth. The profile is asymmetric(approximately sawtooth in form) which leads to a pretilted alignment ifthe nematic is under the influence of a bulk twist torque (seeGB-A-2,296,466; WO-95/22078). These surfaces were constructed into cellsin which the groove direction on one surface was orthogonal to thegroove direction on the other. The cell gap was set to 2.05 μm whichcorresponds to the first Gooch and Tarry minimum when used with E7 (J.Phys. D. Appl. Phys. vol. 8, p. 1575 (1975) ). Filling was then carriedout using E7 in the isotropic phase (65° C.) followed by slow cooling toroom temperature.

The electrooptic response of cells containing different N65/E7 mixturewas then recorded by placing the twisted nematic cell between crossedpolarisers which were oriented parallel to the adjacent gratingalignment directions. Transmission was measured using a photodiode witha photo-optic response during the application of a 1 kHz sinusoidaldrive waveform. FIG. 4 shows the transmission versus rms voltage for twocells one of which was treated to give weak anchoring. The weakanchoring treatment consisted of adding 2% N65 to E7 and curing for 10minutes in a pre-cell before transferring the material to the test cell.The data clearly shows that the weak anchoring treatment has lowered theoperating voltage. A transmission of 50% of the zero volt value isreached at a voltage of 1.83 V for the weak anchored surface and 2.13 Vfor the strong anchored surface. The power consumption of a display canbe considered in the most simple case as the power required to chargeand discharge a capacitor which is proportional to V². Therefore theweak anchored surface is expected to allow a power saving of roughly35%.

The second improvement of the weak anchored cell is the improved opticalcontrast ratio as shown in FIG. 5. At 5 V the weak anchored cell has acontrast ratio of 126 while the strong anchored cell has a contrastratio of 49. At 8 V the difference is even larger (410 and 74respectively). Therefore, a particular application demands a certaincontrast ratio then it can be reached at a much lower voltage with aweak anchored surface. The weak anchoring has also lead to a slightincrease in the steepness of the electrooptic response. For the weakanchored surface V₉₀-V₅₀ is 0.454 V while for the strong surface thisquantity is 0.510 V; V₉₀ and V₅₀ being the voltage at transmissions of90% and 50% of the zero voltage transmission value respectively.

In summary, the above experimental results have shown qualitativeagreement with the theoretical analysis by demonstrating that a surfacewhich has been treated to give a lower W_(θ) can improve a twistednematic device. The improvements include a lower voltage threshold, asteeper electro optic response, and a higher optical contrast.

EXAMPLE TN2

In this example MXM035 (Merck) was used as the weak anchoring treatment.The alignment surfaces were asymmetric monogratings as described inexample TN1.

The MXM035 consists of two parts which were mixed in equal quantities.This mixture was then added to E7 nematic to give a 4% solution whichwas cured in a fused silica cell (as described in example TN1) beforetransferring to a test cell. A measurement of W_(θ) for the 4% solutionrevealed a value of 3.85×10⁻⁴J/m². This is 16 times smaller than thevalue of measured for N65 (in example TN1). Therefore, the MXM 035 tentis expected to have a larger effect on the operating behaviour of a TNdevice.

Cells were constructed using asymmetric monogratings as alignmentsurfaces. The groove direction on one surface was orthogonal to that onthe other to ensure a twisted configuration with a liquid crystal twistof roughly 90°. The cell gap was set to 2.05 μm (the first Gooch andTarry minimum). FIG. 13 shows the transmission versus rms voltage fortwo cells, one of which was treated with 4% MXM035 to give weakanchoring. In this case the weak anchoring treatment has lead to a verylarge decrease in operating voltage. A transmission of 50% of the zerovolt value is reached at a voltage of 0.8 V for the weak anchoredsurface compared to 1.93 V for the strong anchored surface. Hence theweak anchored TN is expected to use only 17% of the operating power of aconventional TN.

Weak anchoring treatments can also lead to improvements in performancewhen the call gap is larger. To demonstrate this, data was taken fromtwo more TN cells which had been constructed with a cell gap of 4.6 μmwhich corresponds to the second Gooch and Tarry minimum for E7. One ofthese was filled with pure E7 while the other was filled with 4% MXM035in E7 which had been precured as described above. FIG. 14 shows theelectrooptic response for these two cells. Once again the weak anchoredcell shows a response at a much lower voltage. The 50% transmissionoccurs at 1.07 V for the weak anchored TN compared to 2.16 V for theconventional TN. The weak anchoring treatments are expected to changethe dynamic response of the TN and so optical response times weremeasured for these cells for switching between 0 V and 4V as shown intable 2. It can be seen that the addition of MXM035 has lead to adecease in switch-on time (τ_(on)) and an increase in switch-off time(τ_(off)). This behaviour is consistent with weak surface anchoring.

TABLE 2 Cell Mixture τ_(on) (ms) τ_(offee) (ms) E7 6.8 15.5 E7 + 4%MXM035 2.5 24.0

0-4 V optical switching times for second minimum TN cells with strong orweak anchoring.

EXAMPLE TN3

Weak anchoring treatments can also be used in conjunction with othersurface alignments to improve TN performance. In this example the MXM035treatment is used in conjunction with a rubbed polymer alignment. Rubbedalignment surfaces were prepared by spin coating a layer of probomide 32(Ciba Geigy) onto ITO coated glass and baking the suites at 300° C. Thesurfaces were then rubbed in one direction by a nylon cloth attached toa rotating roller. Finally cells were constructed in which the rubbingdirection on one surface was orthogonal to that on the other. The cellgap was set to 4.6 μm using monodispersed space beads in the edge seal.FIG. 10 shows a comparison of electrooptic responses recorded from twoTN cells, one filed with E7 and the other filled with E7+4% MX035. Onceagain the addition of the weak anchoring treatment has lead to areduction in operating voltage.

EXAMPLE TN4

One particular type of twisted nematic device is the VCT device, whichswitches from a substantially non-twisted state to a twisted state whena voltage is applied. In this example the operation of the VCT device isimproved by the addition of a weak anchoring treatment. The surfacealignment in this example was provided by asymmetric grating surfaces asdescribed in example TN1.

The weak anchoring treatment was M035 mixed from 10% of part A and 90%of part B. 4% of this mixture was added to nematic MLC 6608 which has anegative dielectric anisotropy. The MXM035 was then cured by placing theMXM0351/MLC6608 mixture in a glass cell and exposing to UV radiation (10minutes at 2.0 mW/cm² while at a temperature of 65° C.). After curing,the mixture was used to fill a VCT test cell.

The VCT cells were constructed so that the grating grooves on onesurface were orthogonal to those on the other. Prior to construction,the grating on one surface was treated with a chrome complex surfactantin order to induce a homeotropic boundary condition (moleculesperpendicular to the wall surface). The grating on the other surface wasleft untreated in order to induce a planar boundary condition. The cellgap was set to 5.3 μm using spacer beads in the edge seal. FIG. 6 showsthe electrooptic response of two VCT devices, one of which contains theweak anchoring treatment The VCT containing pure MLC6608 exhibits 50%transmission at a voltage of 2.91 V while the VCT containing 4% MXM035in MLC 6608 exhibits 50% transmission at a voltage of 1.52 V. Therefore,the weak anchoring treatment has lead to a dramatic decrease inoperating voltage.

The dynamic response times of the VCT were measured for these cells forswitching between 0 V and 5V as shown in table 3. It can be seen thatthe addition of MM035 has lead to a decrease in switch-on time (τ_(on))and an increase in switch-off time (τ_(off)).

TABLE 3 Cell Mixture τ_(on) (ms) τ_(offee) (ms) MSc 6608 86 60 MSC6608 + 60 112 4% MXM035

0-5 V optical switching times for VCT cells with strong or weakanchoring.

The above examples show that the addition of oligomeric materials(Norland 65, MXM035) into cells with either grating surfaces or rubbedpolymer surfaces will lead to a reduction in the operating voltage of atwisted nematic device.

Bistable Nematic Devices

These have substantially the same basic construction as shown in FIGS. 1and 2, with certain differences. The input polariser is parallel to onealignment state for the case where the alignment states differ inazimuthal angle by 90°. The most significant practical difference isthat at least one of the cell walks 3, 4 are treated with alignmentgratings to provide a bistable alignment (rather than a monostablealignment as for the twisted nematic case), i.e., two stable alignmentdirections shown as R,R′ 45° apart, but may be 90°. For example, thealignment may be provided by a bigrating with symmetric and asymmetricprofiles to give both alignment and a required amount of pretilt.Techniques for producing bigratings to give bistable nematic devices aredescribed in GB-A-2,286,467 (PCT-WO-95/22077) and WO97/14990,(PCT-96/02463, GB95 21106.6).

The second surface of the cell (if not provided with a bigrating) may betreated with either a planar or homeotropic monostable surface.

Conventional alignment techniques provide pretilt (zenithal) andalignment direction (azimuthal) with substantial anchoring energy. Thismeans that switching of the device under the influence of electricfields results in movement of the liquid crystal molecules mostly in thecentre of the layer, and zero movement at and adjacent the wall surface.Both pretilt and alignment direction are necessary for good deviceperformance. What is wanted is pretilt values and alignment togetherwith reduced anchoring energy so that molecules at or adjacent the cellwalls can move under the application of normal voltage levels.

The Embodiments of the present invention provide such a wanted pretiltand alignment together with lowered surface anchoring energy. Theinvention does this, in one embodiment, by inclusion of oligomer unitsin the liquid crystal layer 2 and which preferentially migrate to thecell wall surfaces.

Bistable nematic switching relies on surface director reorientation andin order to achieve low voltage switching both the zenithal andazimuthal anchoring energies must be reduced.

Furthermore any additional inelastic memory azimuthal anchoring (P.Vetter et al, Euro Display 1993, SID, p.9) due to microscopic absorptionof the first nematic layer into the surface must be removed. Thisanchoring memory may have the effect of pinning molecules in a givenposition (e.g. between the two switched states) to which they may returnafter removal of a voltage. Ideally, this memory should be completelyremoved so that the molecules remain in their switched positions afterremoval of voltages. In practice, reduction, rather than completeremoval can be satisfactory.

Three examples of the weak anchoring treatment applied to a bistablesurface are now given:

EXAMPLE BN1

The treatment consists of adding a small (1-10%) amount of a UV curingadhesive material to the nematic prior to cell filling. Examples ofsuitable materials include N65, N63, N60 or N123 (All manufactured byNorland Products Incorporated, North Brunswick, N.J., USA). In thisparticular example, one of these materials (N65) is used.

In the first experiment, this material was added to nematic E7 inconcentrations of 1% 2%, 4% and 6%. The mixture was then used to fillcells whose inside walls were coated with flat polymer layers, withoutany alignment direction. The purpose of this experiment was to confirm aweakening of the surface anchoring with the addition of N65. Surfaceswere prepared using a layer of photoresist (Shipley 1805) which washardbaked to 160° C. to ensure insolubility in the liquid crystal.

Cells were made using these surface with a gap of 10 μm. Each was filledwith a different concentration of N65 including a control sample of pureE7. Filling was carried out in the isotropic phase (65° C.) followed byslow cooling to room temperature, and without exposure to UV light. Allcells showed a random planar alignment of the nematic which is alsocalled a Schlerien texture. In the case of the pure E7, 1% N65 and 2%N65, the texture could not be moved by applying finger Pressure to thecell walls whereas for the 4% and 6% mixtures, the texture was highlymobile and domain walls could be easily moved by applying small amountsof pressure. Once the domain walls had been moved they did not return totheir original position but instead remained in the now position forlonger than several days. Therefore, 4% and 6% mixtures lead to a lossof memory anchoring.

A second similar experiment was carried out in which the cells wereexposed to UV radiation after filling but before cooling to roomtemperature. In this case, the 4%, 6% and also the 2% mixtures showedroom temperature domain wall mobility. The improvement in the 2% mixturecan be explained as follows. The N65 material contains both esters andacrylate monomers which polymerise under UV radiation to form oligomerunits which then join together to form larger polymer chains. If the 2%solution is cured for a short time then the reaction can be terminatedwhen only oligomer units have been formed. The oligomers do not phaseseparate from the liquid crystal but they so migrate preferentially tothe surface in order to minimise the surface free energy. This has theeffect of diluting the amount of liquid crystal at the surface whichleads to an effective reduction in the order parameter, S which isdefined by (P. G. deGennes, The Physics of Liquid Crystals, ClarandonPress, Oxford 1974): $\begin{matrix}{S = {\frac{1}{2}\left( \left( {{3\quad \cos^{2}\theta} - 1} \right) \right)}} & (3)\end{matrix}$

The reduction in nematic order due to a oligomer concentration has thetwofold effect of screening the liquid crystal from the surface whichremoves the memory anchoring as well as reducing the elastic anchoringenergy, W_(θ). The uncured material also has this effect but must beadded in a greater concentration (>4%) as the acrylate monomer does notpreferentially migrate towards the surface.

The cured material containing 2% N65 was then removed from the cell inwhich it was cured and used to fill another cell. This second cell alsoshowed a highly mobile Schlerien texture demonstrating that the weakanchoring effect is due to the additive in the bulk as opposed to anysurface layer formed during curing.

The next set of experiments were designed to show the effect of the N65treatment on the zenithal anchoring energy, W_(θ). This quantity can becalculated by measuring the saturation voltage, V_(s) as described inexample TN1.

Results are shown in Table 4. The pure E7 cell failed to show a blackstate before cell breakdown and so only a lower limit on W_(θ) can begiven. In the cases of the E7 containing N65, the curing was performedin a fused silica cell prior to transferring the mixture to a separatemeasurement cell. The exposure was carried out using an unfilteredmercury lamp with an optical output of 2.0 mW/cm².

TABLE 4 Cell mixture Cure time (m) W_(θ)(N m⁻¹) Pure E7 —  >5 × 10⁻² 2%N65 in E7 5 5.4 × 10⁻³ 2% N65 in E7 10 6.3 × 10⁻³ 2% N65 in E7 30 7.7 ×10⁻³ 2% N65 in E7 + pure E7 (1:1) 10 8.0 × 10⁻³

Surface zenithal anchoring energies modified by the presence of N65.

Greater cure times are found to lead to stronger anchoring which isconsistent with the formation of longer polymer chains which tend tophase separate form the nematic rather than lower its surface orderparameter. One set of data shows that the anchoring energy can also beadjusted by diluting the N65/E7 mixture in pure E7. In all cases thepercentage of N65 during the cure process was kept to 2% to ensureconsistent reaction kinetics.

The above results confirm that the N65 treatment leads to a loss ofin-plane memory anchoring as well as lowering (by about an order ofmagnitude) the zenithal anchoring energy. The next stage is to test theeffect of the treatment on the switching of a bistable nematic device.

One example of a surface which can offer bistable nematic alignment is asurface bigrating (as described in GB2286467-A) prepared in a mannersimilar to that shown in FIG. 7. In this case samples were made by spincoating 1805 photoresist 20 onto ITO coated glass 21 at a spin speed of3000 rpm to give a coating thickness of 0.55 μm. The samples were thensoftbaked at 90° C. for 30 minutes. The bigrating was exposed through amask 22 using hard contact photolithography (i.e. normal to the mask 22surface, not at 60° as in FIG. 7) with a typical exposure time of 250 s(at 0.3 mW/cm²). The mask 22 contained a bigrating pattern of 0.9 μmchrome squares separated by 0.5 μm gaps in each direction giving a pitchof 1.4×4 μm. Development was then carried out in Shipley MF319 for 10sec followed by a water rinse. Samples were finally baked at 160° C. for45 minutes after first removing a deep UV exposure to preharden thephotoresist (3.36 J/cm² at 254 nm). This process created a bigratingwith two identical modulations each of which had a symmetric profile.The alignment of a nematic on this is therefore expected to consist oftwo non tilted alignment states (alignment, but no surface tilt)separated by an azimuthal angle of 90°. Such an alignment is notnormally of use in display devices, but was prepared for test andcomparison purposes.

Cells having a cell gap of 0.95 μm were then made with these (zeropretilt) bigratings on both inner surfaces arranged so that the groovedirections on one surface coincided with the groove directions on theother. These were filled with E7 nematic containing variousconcentrations of N65 in its isotropic phase. On cooling to roomtemperature all cells showed two alignment directions as expected. Asthe states have no surface pretilt, and hence no splay, there is nomethod (like the flexoelectric coupling in splayed configurations) bywhich only one state can be selected using applied electrical pulses.However random switching between the states can occur which was seen bypulse-induced domain wall movement.

Rectangular monopolar pulses of various pulse lengths were applied toeach cell. Each pulse alternated in sign from the previous pulse tomaintain a dc balance. Pulses were separated by a time interval 100times the pulse length. For each pulse length a voltage existed abovewhich domain wall movement occurred. FIG. 8 shows this threshold voltageversus time for two cell; one a cell filled with pure E7, and the othera cell filled with a 2% precured mixture of N65 in E7. The N65 hasclearly lowered the voltage threshold to a value of only 5.0 V/μm for a10.8 ms pulse. In contrast, the pure E7 cell shows a much higher voltageswitching (15.0 V/μm) and in fact suffered dielectric breakdown for lowpulse lengths.

The above results show that the N65 treatment lads to a low voltagebistable switching which is consistent with a low zenithal anchoringenergy combined with no in-plane memory anchoring other an the elasticW_(Φ) imposed by the bigrating.

In order to obtain fully selective bistable switching one state has tobe favoured by the applied pulse. This can be achieved using dc couplingto a flexoelectric polarisation if the two bistable states have theappropriate pretilts. In WO 9210054, pretilt can be obtained by usingobliquely evaporated SiO.

A more controllable method, described in GB2286467-A achieves pretilt byusing a bigrating in which both modulations have asymmetric profiles.This method allows a pretilt of typically 17° for one of the bistablestates while maintaining a pretilt of 0° for the other state. Thesesurfaces were tested in conjunction with the N65 treatment using thefollowing fabrication process as illustrated in FIG. 7.

A thin layer 20′ of 1805 photoresist was spun onto ITO coated glass 21′as described above. After softbaking, the layer 20′ was exposed throughthe 1.4×14 μm pitch mask 22′ using an off axis diagonal exposuregeometry as shown in FIG. 7; i.e., exposure at about 60° to the surfacenormal and about 45° to the mask array of square pixels. The exposuretime was set to 540 seconds (at 0.15 mW/cm²). After development andprocessing, the bigrating was constructed opposite a flat photoresistsurface (i.e., no grating and hence zero pretilt) using 10 μm cellspacers to allow measurement of the surface pretilt at the bigratingsurface.

Table 5 shows the pretilt of the tilted state measured by the crystalrotation method (T. J. Scheffer and J. Nebring, J. Appl. Phys., vol.48,no. 5, p. 1783 (1977)) for cells filled with various mixtures. In allcases the non-tilted state had a pretilt of less than 0.1%.

TABLE 5 Mixture used to fill cell Pretilt (°) Pure E7 17.5 2% N65 inE7 + pure E7 (1:1) 17.1 2% N65 in E7 + pure E7 (2:1) 15.4 2% N65 in E72.2

Surface pretilt modified by the presence of N65.

The pure E7 gave a high pretilt as expected but the addition of 2% N65(precured) lead to a catastrophic loss of pretilt which means that thistreatment is not suitable for a flexoelectric coupled device where asignificant pretilt is required in order to achieve de sensitivity.However further diluting of the 2% mixture via addition of pure E7allows the pretilt to be at a value close to the pure E7 cell.

Comparison of data in table 4 and table 5 shows that it is possible toobtain a mix regime which provides weak zenithal anchoring whilemaintaining surface pretilt. The loss of pretilt for 2% N65 in E7 can beunderstood by the further weakening of W_(θ) which allows the nematic toadopt a non tangential orientation with the local surface. Thisdestabilises the high pretilt state with respect to the low pretiltstate. Such states are described in GB9502635.7, “liquid crystal devicealignment”, G. P. Bryan-Brown, C. V. Brown and D. G. McDonnell.

The above results have shown one example of how an oligomeric additive(N65) can be mixed with a typical nematic (e.g BE7) to improve thevoltage response of a bistable nematic device without compromising othersurface parameters such as pretilt.

EXAMPLE BN2

Another example of a weak anchoring treatment was synthesised usingblock polymerisation of a thiol and a vinyl ether to form the structureA shown below, the subscript n is number of repeats in a chain.

—[S(CH₂)₆S CH₂O(CH₂)₆O CH₂CH₂]—  A

As in example BN1, the precursor materials were added to E7 and thencured in a fused silica pre-cell. The cured mixture was then transferredto a second cell whose inner surfaces were coated with hardbakedphotoresist. Filling was carried out at 65° C. followed by slow coolingto room temperature. One particular cell was filled with a mixture whichhad been cured from a 5% solution of the precursor materials in E7. Thiscell showed a schlerien texture with highly mobile domain wallsconfirming that all in-plane memory anchoring had been lost.

A measurement of the saturation voltage revealed that the zenithalanchoring energy had been lowered to a value of 1.2×10⁻³ N m⁻¹. This iseven lower than the values shown in table 4 which indicates that theoligomer units formed in this case are more effective at lowering thesurface order parameter of the nematic phase. Once again dilution of the5% solution into pure E7 revealed a regime on grating surfaces in whichweak zenithal anchoring was combined with high (>15°) pretilt.

Therefore, the structure A is another successful example of an oligomerwhich can be added to a nematic (e.g. E7) to improve the voltageresponse of a bistable nematic device without compromising other surfaceparameters such as pretilt.

EXAMPLE BN3

The following structures B is a list of example monomers which can alsobe used to created weak anchoring treatments.

CH₂═CH O(CH₂)₆O CH═CH₂ HDVE (Hexane -1,6-diol di ...B (vinyl ether)CH₂═CHOC₄H₉ BVE (Butyl vinyl ether) ...B HSCH₂CO₂(CH₂)₂OCOCH S₂H EGTG(Ethylene glycol bis ...B (thioglycollate)) HS(CH₂)₉SH NDT(Nonane-1,9-dithiol) ...B

In previous examples low voltage bistable nematic switching has beenshown to occur when W_(q) is lowered. Therefore, the materials in B havebeen investigated to determine their effect on W_(θ).

Mixtures of these materials were cured before adding to the liquidcrystal and the final mixture was tested in cells containing flathardbaked photoresist on the inner surfaces as described in example BN1.Values of W_(θ) were then obtained using the method described in exampleBN1.

Of the materials listed, EGTG and NDT are monomers with thiolterminations while HDVE and BVE are difunctional and monofunctional‘-ene’ materials, respectively.

The first set of mixtures studied are shown in table 6. In each case apercentage of monofunctional BVE has been added to the bifunctional HDVEin order to induce chain termination and so form oligomers with smallermolecular weights. In each case the quoted percentage is the molarquantity of BVE with respect to HDVE. Furthermore, the quantity of NDTin each mixture was varied to maintain an equal number of thiol groupsand ene groups. To each mixture was added 1% of Igracure 651 (Merck)which acts as a photoinitiator. For each material, curing was carriedout under a mercury lamp (2.0 mW/cm²) for 10 minutes. E7 (Merck) wasused as the liquid crystal to which was added 2% of each material (byweight). The results in table 6 show that the resulting zenithalanchoring energy (W_(θ)) lies in the range 3.6-8.8×10⁻³ J m⁻².Therefore, all the mixtures can be considered to be successful inreducing the anchoring from the value found for pure E7 (>5×10⁻² J m²).Furthermore addition of more BVE and hence shorter oligomer chains isfound to lead to weaker anchoring.

TABLE 6 Material W_(θ)× 10⁻³ J m⁻² NDT/HDVE/2% BVE 8.8 NDT/HDVE/5% BVE5.9 NDT/HDVE/20% BVE 3.6

Measurement of zenithal anchoring for thiol/diene systems with chaintermination.

The correlation of anchoring and molecular length for a given materialtype was further tested using a set of mixtures containing EGTG, HDVEand BVE. In this case GPC analysis was carried out in order to measurethe molecular weights of each material as shown in table 7; Mn is numberaverage of each chain, Mw is average weight per chain, and W_(θ) iszenithal anchoring energy. The smallest portion of BVE (2%) is indeedfound to lead to the longest molecular weights and vice versa. 1% ofeach of these materials was added to E7 and W_(θ) was measured. Thecorrelation of W_(θ) with molecular weight is fairly good consideringthe errors in the W_(θ) measurement.

TABLE 7 Material Mn Mw W_(θ)× 10⁻³ J m⁻² EGTG/HDVE/2% BE 12640 2733010.9 EGTG/HDVE/5% BE 6970 17140 5.3 EGTG/HDVE/10% BE 5000 11550 6.6EGTG/HDVE/20% BE 2900 6200 4.4

Measurement of zenithal anchoring for thiol/diene systems with chaintermination.

To summarise, in this example, two sets of materials have been studiedand both have been found to lead to a reduction in W_(θ). Furthermore,samples of these materials have also been found to reduce the switchingvoltages in bistable nematic devices.

Smectic Devices

The display cell 101 shown in FIGS. 16, 17 comprises two glass walls102, 103 spaced about 1-6 μm apart by a spacer ring 104 and/ordistributed spacers.

Electrode structures 105, 106 of transparent tin oxide are formed on theinner face of both walls. These electrodes are shown as row and columnforming an X, Y matrix but may be of other forms. For example, radialand curved shape for a polar coordinate display, or of segments form fora digital seven bar display, or plain sheet electrodes to form anoptical shutter.

A layer 107 of smectic liquid crystal material is contained between thewalls 102, 103 and space ring 104.

Polarisers 108, 109 are arranged in front of and behind the cell 101.Row 110 and column 111 drivers apply voltage signals to the cell. Twosets of waveforms are generated for supplying the row and column drivers110, 111. A strobe waveform generator 112 supplies row waveforms, and adata waveforn generator 113 supplies ON and OFF waveforms to the columndrivers 111. Overall control of timing and display format is controlledby a control logic unit 114.

Prior to assembly, the walls 102, 103 are surface treated by spinning ona thin layer of polymeric material such as polyimide or polyamide,drying and where appropriate curing; then buffing with a soft cloth(e.g., rayon) in a single direction R₁, R₂. This known treatmentprovides a surface alignment for liquid crystal molecules. In thenematic and cholesteric phases and in the absence of an applied electricfield the molecules at the surface walls 102, 103 align themselves alongthe rubbing direction R₁, R₂ and at a pretilt angle ξ of about e.g., 2°to 10° to the surface.

The surface alignment treatment is arranged to provide the requiredvalue of pretilt ξ. For example, material polyimide (e.g., Polyimide 32)when rubbed gives a typical pretilt of about 2°; the actual valuedepends upon liquid crystal material and the processing. Alternatively,as described in GB-A-2,286; GB-A-2,286,467; GB-A-2,286,894;GB-A-2,2986.893, the cell walls may have formed thereon gratingstructures which provide a range of pretilt angles and alignmentdirections. The gratings may be symmetric and/or asymmetric in profile,and shaped to give any desired value of pretilt ξ, and azimuthal andzenithal anchoring energies β, α respectively.

The device may operate in a transmissive or reflective mode. In theformer, light passing through the device e.g., from a tungsten bulb 115is selectively transmitted or blocked to form the desired display. Inthe reflective mode, a mirror 116 is placed behind the second polariser109 to reflect ambient light back through the cell 101 and twopolarisers. By making the mirror 116 partly reflecting, the device maybe operated both in a transmissive and reflective mode.

Pleochroic dyes may be added to the material 107. In this case only onepolariser is needed and the layer thickness may typically be 4-10 μm.

If the smectic material 107 is a chiral smectic e.g., smectic C (S_(c)*)then a bistable device can be made. Such a device is the surfacestabilised ferroelectric device (SSFLC) supporting two bistable stateswhich are optically distinct. In a chiral smectic material, moleculestend to lie and move along the surface of an (imaginary) cone as shownin FIG. 9. When the surface alignment directions R1, R2 are parallel the(z) axis of these cones are parallel to these alignment directions andthe molecules lie on either side of the axis on the cone surface.

In one switched state, D1, the molecules lie on one side of the cone,and in the second bistable state, D2, lie on the other side of the cone.The switching is achieved by application of a voltage pulse ofappropriate sign and length applied through the electrodes 106, 107coupling with a spontaneous polarisation coefficient Ps of the material.The cone angle, θ_(c), is a function of material parameters. In devices,the molecules in their two switched positions D1, D2, do not lie on theextremities of the cone but some small distance away. This means thatthe angle between the bistable positions is somewhat less than the coneangle, and can be increased a bit by application of an ac voltage signalto the material. This is known as ac stabilisation mentioned above.Ideally, the angle between the switched states is 45° because this wouldallow maximum contrast for the cell when arranged between crossedpolarisers 108, 109 with the axis of one polariser along one of theswitched directions. This gives a dark state in one switched positionand a light state in the other switched position.

The angular distance between the two states is defined as the memoryangle, θ_(m) (see N. Itoh et al, Jpn. J. Appl. Phys., 31, L1089 (1992)).The optimum memory angle for maximum brightness in the light state istherefore 45°. However, most materials possess a memory angle which ismuch less than 45° and so suffer from loss in brightness.

A weak anchoring treatment can be added to a ferroelectric to increasethe memory angle and so improve the display brightness. This treatmentalso allows small amounts of translational movement of microlayersformed during cooling from isotropic phases to smectic phases leading toimproved alignment.

Example of Cell Preparation

Alignment surfaces were prepared by spin coating a layer of probomide 32(Ciba Geigy) onto ITO coated glass and baking the substrates at 300° C.The surfaces were then rubbed in one direction R by a nylon clothattached to a rotating roller. Finally cells were constructed in whichthe rubbing direction R₁ on one surface was parallel to that R₂ on theother. The cell gap (d) was set to 1.1 μm using monodispersed spacerbeads in the edge seal. Each cell was then filled with ZLI 5014 (Merck)ferroelectric liquid crystal doped with small percentages of N65. Beforefilling the N65 was cured in a separate cell.

FIG. 10 shows the memory angle measured from two cells containing eitherpure ZLI 5014, or 4% N65 in ZLI 5014 as a function of applied voltage(50 kHz AC). The results clearly show that the weak anchoring treatmenthas lead to a significant increase in memory angle at all voltages.Therefore, the treatment has improved this ferroelectric device byincreasing the ON state transmission between crossed polarisers. Forexample, at 5 V, the memory angle has increased from 17.1° to 34.4°which would lead to a device which is 3.7 times brighter.

Bistable ferro electric devices switch upon receipt of a unidirectionalpulse of appropriate direction, amplitude, and length. Strobe pulses areapplied sequentially down the rows, whilst one of two different datapulses are applied to each column. Examples of addressing are describedin U.S. Pat. No. 5,497,173, GB2,232,802; U.S. Ser. No. 07/977,442,GB-2,262,831.

Several other smectic devices may be made with alignment surfaces of thepresent invention. For examples: electro-clinic smectic devices; monostable ferro electric devices U.S. Pat. Nos. 5,061,047, 4,969,719,4,997,264, colour change smectic projection cells U.S. Pat. No.5,189,534, GB2,236,403. The alignment may produce a chevron type C1 orC2 type of smectic micro layer arrangement; or a tilted bookshelfarrangement where rubbing directions on opposite walls are in the samedirection, or real bookshelf alignment.

Reducing anchoring energy allows small amounts of translational movementto occur in micro layers formed during cooling from isotropic phases tosmectic phases.

Reduction of anchoring energy can be applied to various smectic devicesas follows:

(i) Bookshelf and Quasi-book-shelf with low surface viscosity and nosurface memory effects.

Most materials used in FLC devices exhibit layer shrinkage on coolingthrough the SmC* phase due to the increase of the angle between themolecules and layer normal on cooling. The tendency for the pinning ofthe smectic layers at the surfaces then leads to the formation of achevron structure.

If the pinning energy is sufficiently high to prevent any translationalslippage of the layers (i.e., the energy cost associated with layerslippage is much greater than energies associated with the chevroninterface, elastic distortion of the director in the triangular directorprofile, and the orientational surface energy associated with thedirector being unable to lie in the preferred alignment direction) thelayer shrinking requires that the layers tilt with respect to thesurface normal.

If both surfaces have similarly high layer slipping terms then thelayers must tilt into a chevron structure which is necessarily symmetricabout the central plane of the cell. For typical materials, this degreeof layer shrinkage is such that the layer tilt angle δ is a constantfraction of the smectic cone angle θ usually about δ/θ=0.85. This causesa reduction in the angle between the two bistable states and henceoptical contrast in the cell. A higher memory angle may be achieved bylowering δ in what is often termed quasi bookshelf geometry. If thelayer pinning term is made sufficiently weak (for example a relativelyhigh concentration of the surfactant is used) a bookshelf geometry isobtained i.e., δ=0. If the azimuthal angle β is also made sufficientlylow then a uniform director profile is possible in which the twobistable states are at the optimum angle of ±θ to the rubbing direction.The resulting high contrast and brightness of the display is alsocombined with the other advantages of reduced/no surface memory effects(which would be a problem in other bookshelf devices) and fasterresponse (due to the decoupling of the surface director from the solidsurface the surface viscosity becomes equivalent to that of the bulk).

(ii) Chevron with improved memory angle.

This device uses the slippery surfactant at sufficient concentration tolower layer tilt angle in chevron geometry, thereby leading to a highermemory angle and improved brightness for multiplexed devices. However,it may be possible to lower the orientational surface energies without astrong effect on the translation energy (i.e., layer pinning). Thus, thechevron structure would remain to a large extent (i.e., δ remainsunchanged) but the orientation of the director at the surface would behigher. For no applied AC field the surface twist of the director wouldapproach that of the chevron interface. The optical uniformity of thisstate (and hence contrast) would be improved. Moreover, the loweredsurface energy would increase the angle of the director at the surfacewith an applied AC field, and thus the brightness of an AC stabiliseddisplay will also be enhanced.

(iii) Improving isotropic to smectic transition.

Surfactant allows layers to slip easily over the surface to form anenergy state (i.e., uniform layers) dictated by orientational propertiesof the surface alone (i.e., no translational restrictions). Particularlyuseful also in AFLC where N* (cholesteric) phase is usually notapparent, but also in other devices (e.g., FLC) where stringent materialrequirements prevent the use of an N(*) phase.

(iv) Improved stability to mechanical, triennial or electrical damage ofsmectic devices.

Disruption of a well aligned smectic sample though mechanical,electrical or thermal shock leads to pinning of the layers at thesurface which is difficult to remove, even though the disrupted state isnot the lowest energy state. If the pinning is removed then the systemmay relax back to this minimum energy state before the disruption.

(v) Improved high tilt chevron device.

High surface pre-tilts are used to ensure that the surface orientationof a FLC device in the chevron geometry approaches the cone angle andhence the memory angle is improved (this is used by CANON and is inJones, Towler Hughes review). In the unwound N* phase the director has alarge degree of splay and bend distortion. On cooling into the smectic Aphase this bend cannot be supported, due to the presence of the layersand the distortion is pushed to the surfaces where the director isforced to lie away from the desired pretilt. This may lead to variationsin the alignment and hence defects in the SmC* phase (until switchedthis geometry often forms a “sandy texture” on first cooling). Moreover,there may be a plastic change of the pre-tilt caused by the zenithalsurface memory effect. This means that when cooled into the SmC* phasethe effective pretilt is lowered and the resulting memory angle isreduced somewhat.

With the slippery surfactant, the surface memory is reduced and thepre-tilt remains unchanged Note, this is an example where the slipperysurface technique is used in the nematic phase, but results in improvedperformance of a smectic device.

(vi) Improved Electroclinic and antiferroelectric (AFLC) devices

In both of these devices, director twist is induced by the DC electricfield there is a tendency for the smectic layers to shrink. If pinned atthe surface the applied field tends to induce layer tilt, (although incontradiction to the requirement that E ∥ Pi) which reduces opticalappearance through defects, and may also reduce viewing angle since thedirector may tilt out of the cell plane. With the slippery surfactant,both the director and the layers can move easily across the surface,without surface memory or viscosity effects.

(viii) Improved N-SmC device.

Advantages ranging from: reduced tendency to form chevron typestructure; reduced surface memory; faster surface switching.

(viii) Reduced tendency form T state formation in FLC

Due to reduced polar surface interaction. This ensures the good opticaland electro-optical properties of any of the above devices (inparticular the chevron and bookshelf devices).

(ix) Improved alignment of smectic devices.

Treatment prevents defects (for example, pitch lines in overlying N*phase, or C1 state/zig-zags in SmC*) from becoming pinned at surfaceirregularities.

The monomer materials used in embodiments of the invention may includethe following, which are given only by way of example:

methyl acrylate propane-1,3-diol diacrylate ethyl acrylatebutane-1,4-diol diacrylate propyl acrylate pentane-1,5-diol diacrylatebutyl acrylate hexane-1,6-diol diacrylate pentyl acrylateheptane-1,7-diol diacrylate 2-methylbutyl acrylate octane-1,8-dioldiacrylate hexyl acrylate nonane-1,9-diol diacrylate heptyl acrylatedecane-1,10-diol diaclylate octyl acrylate glycerol triacrylate nonylacrylate trimethylolpropane triacrylate decyl acrylate pentaerythritoltriacrylate ethyl hexyl acrylate pentaerythritol tetraacrylate methylmethacrylate di-pentaerythritol hexaacrylate ethyl methacrylate ethyleneglycol dimethacrylate propyl methacrylate 1,2-propylene glycoldimethacrylate butyl methacrylate propane-1,3-diol dimethacrylate pentylmethacrylate butane-1,4-diol dimethacrylate 2-methylbutyl methacrylatepentane-1,5-diol dimethacrylate hexyl methacrylate hexane-1,6-dioldimethacrylate heptyl methacrylate heptane-1,7-diol dimethacrylate octylmethacrylate octane-1,8-diol dimethacrylate nonyl methacrylatenonane-1,9-diol dimethacrylate decyl methacrylate decane-1,10-dioldimethacrylate ethyl hexyl methacrylate glycerol trimethacrylate styrenetrimethylolpropane trimethacrylate ethylene glycol diacrylatepentaerythritol trimethacrylate 1,2-propylene glycol diacrylatepentaerythritol tetramethacrylate di-pentaerythritol hexamethacrylate

A further class of polymers includes di-thiol/diene polymers prepared bythe copolymerisation of difunctional alkenes with difunctional thiolsunder free radical conditions. Monofunctional and/or polyfunctionalalkenes and/or thiols may be incorporated in order to modify theproperties of the polymer, for example to reduce the molecular weight ofthe polymer or to introduce a controlled degree of crosslinking in thepolymer. The following materials given by way of example only may beincluded in polymers suitable for use in embodiments of the invention:

methyl acrylate styrene ethyl acrylate ethylene glycol diacrylate propylacrylate 1,2-propylene glycol diacrylate butyl acrylate propane-1,3-dioldiacrylate pentyl acrylate butane-1,4-diol diacrylate 2-methylbutylacrylate pentane-1,5-diol diacrylate hexyl acrylate hexane-1,6-dioldiacrylate heptyl acrylate heptane-1,7-diol diacrylate octyl acrylateoctane-1,8-diol diacrylate nonyl acrylate nonane-1,9-diol diacrylatedecyl acrylate decane-1,10-diol diacrylate ethyl hexyl acrylate glyceroltriacrylate methyl methacrylate trimethylolpropane triacrylate ethylmethacrylate pentaerythritol triacrylate propyl methacrylatepentaerythritol tetraacrylate butyl methacrylate di-pentaerythritolhexaacrylate pentyl methacrylate ethylene glycol dimethacrylate2-methylbutyl methacrylate 1,2-propylene glycol dimethacrylate hexylmethacrylate propane-1,3-diol dimethacrylate heptyl methacrylatebutane-1,4-diol dimethacrylate octyl methacrylate pentane-1,5-dioldimethacrylate nonyl methacrylate hexane-1,6-diol dimethacrylate decylmethacrylate heptane-1,7-diol dimethacrylate ethyl hexyl methacrylateoctane-1,8-diol dimethacrylate ethylene glycol divinyl ethernonane-1,9-diol dimethacrylate 1,2-propylene glycol divinyl etherdecane-1,10-diol dimethacrylate propane-1,3-diol divinyl ether glyceroltrimethacrylate butane-1,4-diol divinyl ether trimethylolpropanetrimethacrylate pentane-1,5-diol divinyl ether pentaerythritoltrimethacrylate hexane-1,6-diol divinyl ether pentaerythritoltetramethacrylate heptane-1,7-diol divinyl ether di-pentaerythritolhexamethacrylate octane-1,8-diol divinyl ether ethylene glycol diallylether nonane-1,9-diol divinyl ether 1,2-propylene glycol diallyl etherdecane-1,10-diol divinyl ether propane-1,3-diol diallyl ether glyceroltrivinyl ether butane-1,4-diol diallyl ether trimethylolpropane trivinylether pentane-1,5-diol diallyl ether divinyl benzene hexane-1,6-dioldiallyl ether butane-1,3-diene heptane-1,7-diol diallyl etherpentane-1,4-diene octane-1,8-diol diallyl ether hexane-1,5-dienenonane-1,9-diol diallyl ether heptane-1,7-diene decane-1,10-diol diallylether octane-1,7-diene glycerol triallyl ether nonane-1,8-dienetrimethylolpropane triallyl ether decane-1,9-diene di-allyl malonateethylene glycol dithioglycollate di-allyl succinate 1,2-propylene glycoldithioglycollate di-allyl glutanate propane-1,3-diol dithioglycollatedi-allyl hexane-1,6-dicarboxylate butane-1,4-diol dithioglycollatedi-allyl keptane-1,7-dicarboxylate pentane-1,5-diol dithioglycollatedi-allyl octane-1,8-dicarboxylate hexane-1,6-diol dithioglycollatedi-allyl nonane-1,9-dicarboxylate heptane-1,7-diol dithioglycollatedi-allyl decane-1,10-dicarboxylate octane-1,8-diol dithioglycollatedi-allyl undecane-1,11- dicarboxylate nonane-1,9-diol dithioglycollatedi-allyl dodecane-1,12- dicarboxylate decane-1,10-diol dithioglycollatedi-allyl phthalate glycerol trithioglycollate butane-1,4-diol di-3-mercaptopropionate trimethylolpropane trithioglycollate pentane-1,5-dioldi-3- mercaptopropionate pentaerythritol trithioglycollatehexane-1,6-diol di-3- mercaptopropionate pentaerythritoltetrathioglycollate heptane-1,7-diol di-3- mercaptopropionatedi-pentaerythritol hexathioglycollate octane-1,8-diol di-3-mercaptopropionate 4,4′-thiobisbenzenethiol nonane-1,9-diol di-3-mercaptopropionate di-allyl iso-phthalate decane-1,10-diol di-3-mercaptopropionate di-allyl terephthalte glyceroltri-3-mercaptopropionate ethane dithiol trimethylolpropane tri-3-mercaptopropionate propane dithiol pentaerythritol tri-3-mercaptopropionate butane dithiol pentaerythritol tetra-3-mercaptopropionate pentane dithiol di-pentaerythritol hexa-3-mercaptopropionate hexane dithiol Also commercial polymers from Norlandand Merck eg Norland 65, Norland 63 and Merck MXM035 heptane dithioloctane dithiol nonane dithiol decane dithiol undecane dithiol dodecanedithiol ethylene glycol di-3-mercaptopropionate 1,2-propylen glycoldi-3-mercaptopropionate propane-1,3-diol di-3-mercaptopropionate

What is claimed is:
 1. A liquid crystal device comprising: a layer of aliquid crystal material contained between two spaced cell wall carryingelectrodes structures and an alignment treatment on at least one wall,characterized by means for reducing anchoring energy at the surfacealignment on one or both cell walls, comprising: an oligomer or polymerwithin the liquid crystal material at the cell walls.
 2. The device ofclaim 1 wherein the means for reducing energy is an oligomer containingesters, thiols, and/or acrylate monomers within the liquid crystalmaterial at the cell walls.
 3. The device of claim 1 wherein theoligomer or polymer has imperfect solubility in the liquid crystalmaterial.
 4. The device of claim 1 wherein the oligomer or polymer has aphysical affinity for the surface of the cell wall.
 5. The device ofclaim 1 wherein the oligomer or polymer retains a substantially liquidlike surface at the polymer and liquid crystal material interface. 6.The device of claim 1 wherein the oligomer or polymer is substantiallynon-crystalline within the liquid crystal material.
 7. The device ofclaim 1 wherein the oligomer or polymer reduces the liquid crystalmaterial order parameter at or adjacent the cell walls.
 8. The device ofclaim 1 wherein the oligomer or polymer changes the phase of the liquidcrystal material at or adjacent the cell walls.
 9. The device of claim 1wherein the oligomer or polymer has a glass transition temperature belowthe operating temperature range of the device.
 10. The device of claim 1wherein the oligomer or polymer is substantially linear or includesbranch points, either with or without crosslinking.
 11. The device ofclaim 1 wherein the oligomer or polymer has a number of repeat unitswithin the range of 4 to
 1000. 12. A method of making a liquid crystaldevice comprising the steps of: providing a layer of a liquid crystalmaterial contained between two spaced cell wall carrying electrodesstructures and an alignment treatment on at least one wall,characterized by the step of reducing anchoring energy at the surfacealignment on one or both cell walls, providing an oligomer or polymerwithin the liquid crystal material at the cell walls.
 13. The method ofclaim 12 wherein the oligomer or polymer is formed by polymerization ofreactive low molecular weight materials in solution in the liquidcrystal material.
 14. The method of claim 12 wherein the oligomer orpolymer is formed by polymerization of reactive low molecular weightmaterials in solution in the liquid crystal material prior itsintroduction between the cell walls.
 15. The method of claim 12 whereinthe oligomer or polymer is formed by polymerization of reactive lowmolecular weight materials in solution in the liquid crystal materialafter to its introduction between the cell walls.
 16. The method ofclaim 12 wherein the oligomer or polymer is formed by polymerization ofreactive low molecular weight materials in the presence of an inertsolvent which is then removed and the resulting polymer dissolved in theliquid crystal material prior to its introduction between the cellwalls.
 17. A twisted nematic liquid crystal device capable of beingswitched from a twisted state to a non twisted state comprising: twocell walls enclosing a layer of nematic liquid crystal material;electrode structures on both walls for applying an electric field acrossthe liquid crystal layer; a surface alignment on both cell wallsproviding alignment direction to liquid crystal molecules and arrangedso that a twisted nematic structure is formed across the liquid crystallayer; means for distinguishing between the two different optical statesof the liquid crystal material; characterized by means for reducingzenithal anchoring energy in the surface alignment on one or both cellwalls, comprising: an oligomer or polymer within the liquid crystalmaterial at the cell walls.
 18. The device of claim 17 wherein the meansfor reducing zenithal anchoring energy is an oligomer which is coatedonto the inner surface of one or both cell walls either spread on thesurface or added to the liquid crystal material.
 19. The device of claim18 wherein the means for reducing zenithal anchoring energy is anoligomer incorporated in the liquid crystal material.
 20. The device ofclaim 17 wherein the means for reducing zenithal anchoring energy isN65, or MXM035.
 21. The device of claim 17 wherein the means forreducing zenithal anchoring energy is a material containing esters,thiols, and/or acrylate monomers.
 22. The device of claim 17 wherein themeans for reducing zenithal anchoring energy reduces the liquid crystalmaterial order parameter at or adjacent the cell walls.
 23. The deviceof claim 17 wherein the means for reducing zenithal anchoring energychanges the phase of the liquid crystal material at or adjacent the cellwalls.
 24. The device of claim 17 including means for reducing azimuthalanchoring energy.
 25. The device of claim 17 the surface alignmentprovides a pretilted nematic alignment on both cell walls.
 26. Thedevice of claim 17 wherein the surface alignment is provided by a rubbedpolymer, a photo-ordered polymer or an obliquely evaporated inorganicmaterial.
 27. The device of claim 17 wherein the surface alignment layeris a surface monograting with an asymmetric groove profile.
 28. Thedevice of claim 17 wherein the alignment directions on the two surfacesare substantially perpendicular.
 29. The device of claim 17 wherein theliquid crystal director twists by about 90° throughout the thickness ofthe cell.
 30. The device of claim 17 wherein the liquid crystal directortwists is greater than 180° and less than 360°.
 31. The device of claim17 wherein the nematic liquid crystal material contains a small amount(<5%) of a chiral dopant material.
 32. A bistable nematic liquid crystaldevice capable of being switched into two different stable statescomprising: two cell walls enclosing a layer of nematic, liquid crystalmaterial; electrode structures an both walls; a surface alignment on oneor both cell walls providing two alignment directions to liquid crystalmolecules with an amount of surface pretilt; means for distinguishingbetween switched states of the liquid crystal material; characterized bymeans for reducing inelastic azimuthal memory anchoring energy in thesurface alignment on one or both cell walls, comprising: an oligomer orpolymer within the liquid crystal material at the cell walls.
 33. Thedevice of claim 32 and including means for reducing zenithal anchoringenergy.
 34. The device of claim 32 wherein the means for reducing theanchoring energy is an oligomer or polymer which is either spread on thesurface or added to the liquid crystal material.
 35. The device of claim34 wherein the oligomer is a material selected from: Norland N65—[S(CH₂)₆SCH₂CH₂O(CH₂)₆OCH₂CH₂]_(n)— HDVE (Hexane-1,6-diol di(vinylether)) CH₂═CHO(CH₂)₆OCH═CH₂ CH₂═CHOC₄H₉ BVE (Butyl vinyl ether)HSCH₂CO₂(CH₂)₂OCOCHS₂H EGTG (Ethylene glycol bis(thioglycollate))HS(CH₂)₉SH NDT (Nonane-1,9-dithiol).


36. The device of claim 34 wherein the oligomer is an amount up to 10%by weight in the liquid crystal material.
 37. The device of claim 34wherein the chain length (n) is less than 100 repeat units.
 38. Thedevice of claim 34 wherein the oligomer's parameters of type,concentration, and chain length, are arranged to reduce the liquidcrystal order parameter at or adjacent the cell wall.
 39. The device ofclaim 34 wherein the oligomer's parameters of type, concentration, andchain length, are arranged to change the phase of the liquid crystalmaterial at or adjacent the cell wall.
 40. The device of claim 34wherein the oligomer is a material that has been precured prior tointroduction between the cell walls.
 41. The device of claim 34 whereinthe oligomer is a material that has been precured after introductionbetween the cell walls.
 42. The device of claim 32 wherein the surfacealignment is provided by a bigrating surface.
 43. A smectic liquidcrystal device comprising: a liquid crystal cell including a layer ofsmectic liquid crystal material contained between two walls bearingelectrodes and surface treated to give both an alignment and a surfacetilt to liquid crystal molecules; characterized by means for reducinganchoring energy at the surface alignment on one or both cell walls,comprising: an oligomer or polymer within the liquid crystal material atthe cell walls.
 44. The device of claim 43 wherein the means forreducing energy is an oligomer containing esters, thiols, and/oracrylate monomers within the liquid crystal material at the cell walls.45. The device of claim 43 wherein the oligomer or polymer has imperfectsolubility in the liquid crystal material.
 46. The device of claim 43wherein the oligomer or polymer has a physical affinity for the surfaceof the cell wall.
 47. The device of claim 43 wherein the oligomer orpolymer retains a substantially liquid like surface at the polymer andliquid crystal material interface.
 48. The device of claim 43 whereinthe oligomer or polymer is substantially non-crystalline within theliquid crystal material.
 49. The device of claim 43 wherein the oligomeror polymer reduces the liquid crystal material order parameter at oradjacent the cell walls.
 50. The device of claim 43 wherein the oligomeror polymer changes the phase of the liquid crystal material at oradjacent the cell walls.
 51. The device of claim 43 wherein the liquidcrystal material is a chiral smectic material, the alignment directionson the two cell walls are substantially parallel, and the device is abistable device.
 52. The device of claim 43 wherein the alignmentdirections on the two cell walls are non parallel.
 53. The device ofclaim 43 wherein the liquid crystal material is a non-chiral smecticmaterial.
 54. The device of claim 43 wherein the liquid crystal materialis a smectic A material.
 55. The device of claim 43 wherein thealignment is provided by a grating surface.
 56. The device of claim 43wherein the alignment is provided by a rubbed polymer surface.
 57. Thedevice of claim 43 wherein one cell wall has an alignment treatment, theother cell wall has no azimuthal alignment direction, and both cellwalls are treated with the means for reducing anchoring energy.